1. Field of the Invention
This invention relates to an electronic torque wrench equipped with strain gages placed on a part forming a sensor and supplying an output voltage which, depending on the torque exerted on the wrench, reacts on electronic means indicating this torque.
2. The Prior Art
In accompanying FIG. 1, which partially and very diagrammatically represents an electronic torque wrench of this type, 1 is the part forming the sensor, 2 the hollow handle closed by a removable plug 3, and 4 the electronic measuring and display circuit which is associated with strain gages 5, 6 placed on sensing part 1 on both sides of crosswise plane 7. Such a wrench is, for example, structurally such as described in document FR-A-2 568 009. In the drawing, the removable driving head, which fits on part 1 to exert the rotation torque at point A, is not represented.
Under normal conditions of using this wrench, the operator, to apply a rotation torque at A, exerts a force F at determined point B of the wrench. The torque actually measured by the strain gages 5 and 6 is then bending torque C' at point D which is located at an axial distance L' from the point of application of force F. Torque C is deduced from measured torque C' by the relation: EQU C=C'.multidot.(L/L')
In case the operator applies force F at a point B' of handle 2 which, for example, is closer to point A by an axial distance x, the torque displayed by the wrench will be equal to the torque measured at D, multiplied by the preceding factor L/L', since the electronic measuring and display circuits contained in 4 are consequently adjusted. This measurement is inexact since in reality, the torque which should be displayed should be equal to the torque actually measured by gages 5 and 6, multiplied by the factor (L-x)/(L'-x) and not by the factor L/L'.
There has been shown in accompanying FIG. 2 another case of poor use of this wrench. In this case, the operator applies, besides force F directed downwardly and applied at point B with four fingers but not the thumb, a parasitic force F' equal to F but oriented upwardly and located at a point E closer to A. In this case, torque C applied at A is zero and on this point is exerted a tangential displacement force f, applied upwardly. Point A being stationary by definition, the equilibrium of the wrench is reflected by a reaction force R at A, this force being equal and opposite to force f.
At previously defined measuring point D, strain gages 5 and 6 still measure a non-zero bending, so that the value of the torque displayed by the wrench is not zero, yet which should be the case since the actual torque to measure is indeed zero.
In case F is greater than F', there will indeed be an appearance of a torque at A, but the measurement of this torque will be vitiated by a systematic error due to the parasitic bending generated by the application of "parasitic" force F'.
As prior art there can be cited the documents DEA 3139374 and U.S. Pat. No. 4,006,629.